Direct sol-gel fabrication of patterned zirconia thin films using self-assembly in phase-separated mixed Langmuir-Blodgett films

Article abstract of DOI:10.1246/cl.2007.1356

We demonstrate a new method through sol-gel processes for the preparation of patterned zirconia thin films by using directed self-assembly into patterns reflecting the phase-separated structures of mixed Langmuir-Blodgett films. Atomic force microscopy and Kelvin probe force microscopy images confirmed the formation of patterned zirconia thin films. Copyright

Full text of DOI:10.1246/cl.2007.1356

1356
Chemistry Letters Vol.36, No.11 (2007)
Direct Sol–Gel Fabrication of Patterned Zirconia Thin Films Using Self-assembly
in Phase-separated Mixed Langmuir–Blodgett Films
Hirobumi Shibata,Ã1 Masahiro Sato,1 Hideki Sakai,2;3 Masahiko Abe,2;3 and Mutsuyoshi Matsumoto1;3
1Faculty of Industrial Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda 278-8510
2Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda 278-8510
3Institute of Colloid and Interface Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601
(Received August 22, 2007; CL-070899; E-mail: shibata@rs.noda.tus.ac.jp)
We demonstrate a new method through sol–gel processes
muir film was transferred at 10 mN/m using the vertical dipping
method at a withdrawal speed of 5 mm/min onto oxidized Si
wafers. Templates were prepared according to the method
described in a previous paper.9 The surface of the templates
has silanol groups confined in domains that are surrounded by
the SAM of F8H2SiCl. The surface silanol groups were reacted
with EDA–Si to functionalize the templates with amino groups.
Zirconia/SAM composite films were prepared by immersing
the functionalized templates in 360 mM aqueous zirconium sul-
fate (Zr(SO4)2) solution at room temperature for 6 h. The sam-
ples were calcined at 1073 K for 6 h to obtain zirconia thin films.
Structural properties of the obtained films were investigated by
atomic force microscopy (AFM) and Kelvin probe force micros-
copy (KFM) observations.
Figure 1 shows AFM and KFM images of a functionalized
template and a zirconia/SAM composite film. The AFM and
KFM images of the functionalized template show a structure
similar to that of an as-deposited LB film with circular domains
of the size of 1–5 mm. The domain region is lower than the rest.
This is consistent with the film structure in which the domains
consist of the SAM of EDA–Si and the other part is comprised
of the SAM of F8H2SiCl. The surface potential of the domains
is larger than that of the SAM of F8H2SiCl because the surface
for the preparation of patterned zirconia thin films by using di-
rected self-assembly into patterns reflecting the phase-separated
structures of mixed Langmuir–Blodgett films. Atomic force mi-
croscopy and Kelvin probe force microscopy images confirmed
the formation of patterned zirconia thin films.
A sol–gel method has been used to prepare ceramics and
is suitable for controlling the shapes of ceramics to obtain parti-
cles,1 thin films,2 and fibers.3 Metal oxide thin films have attract-
ed much attention as promising materials for electrodes4 and
catalysts.5,6 Thin films obtained through conventional sol–gel
processes are uniformly formed on the substrates, which shows
that one-step patterning is very hard.
The Langmuir–Blodgett (LB) technique is one of the bot-
tom-up technologies and has been used to fabricate organic ul-
trathin films with well-defined structures.7 In particular, phase
separation often occurs in mixed LB films and leads to the
two-dimensional confinement of one component on a microme-
ter or nanometer length scale, which will be useful for high-den-
sity recording and sensing with high sensitivity.8
We have recently succeeded in the preparation of templates,
the patterns of which reflect the phase-separated structures of
mixed LB films.9 In addition, organic and inorganic material in-
cluding gold colloidal particles and copper have been patterned
on the templates.10 When this technique is applied for sol–gel
processes, various inorganic thin films will be patterned for
use in memories, switches, electrodes, and sensors through
one-step sol–gel systems.
(a)
(b)
Here, we report a new method for directed self-assembly of
inorganic films, reflecting the phase-separated structures of
mixed LB films, through a sol–gel route using metal ions as
precursors. We have employed zirconia (zirconium dioxide)
because of their possible applications to catalysts.6
Amphiphilic molecules used in this study are eicosanoic
acid (H19A; Acros Organics), (heptadecafluoro-1,1,2,2-tetrahy-
drodecyl)trichlorosilane (F8H2SiCl; Gelest) and N-(2-aminoeth-
yl)-3-aminopropyltrimethoxysilane (EDA-Si; Gelest). The num-
bers following ‘‘H’’ and ‘‘F’’ denote the chain length of hydrocar-
bon and perfluorocarbon, respectively. Hexane was purchased
from Dojindo. Zirconium sulfate tetrahydrate (Kanto Chem.
Co., Inc.) was used as a zirconia precursor.
(c)
(d)
A Lauda film balance was used for preparation of mixed LB
films. A spreading solution at a concentration of 1:0 Â 10À3 M
was spread on an aqueous subphase. The mixing ratio of
H19A to F8H2SiCl was 1 to 9. The subphase temperature was
293 K. Molecules were compressed at a speed of 1:2 Â 10À2
nm2/moleculeÁmin after 30 min of evaporation time. The Lang-
Figure 1. AFM ((a) and (c)) and KFM ((b) and (d)) images
(20 Â 20 mm2) of functionalized templates before ((a) and (b))
and after ((c) and (d)) immersion in aqueous Zr(SO4)2 solution
at room temperature for 6 h.
Copyright ꢀ 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.11 (2007)
1357
(a)
(b)
(a)
(b)
Figure 2. (a) AFM and (b) KFM images (5 Â 5 mm2) of a
functionalized template immersed into a zirconia precursor,
following by calcination at 1073 K for 6 h.
Figure 3. (a) AFM and (b) KFM images (5 Â 5 mm2) of a zirco-
nia thin film. The template was fabricated from a mixed LB film
of H17A, F8H8A, and F8H2SiOEt at mixing ratios of 2, 2 to 5.
of the SAM of F8H2SiCl is covered with fluorine atoms with
large electronegativity. After immersion in aqueous Zr(SO4)2
solution, the AFM image (Figure 1c) shows that the domains
have locally higher points, indicating the confinement of materi-
al in the domains. The KFM image of this film suggests that
these circular domains are covered with a single component
because the observed domains have almost the same surface
potential. These results suggest that a zirconia precursor is
confined in the circular domains.
aqueous zirconium sulfate solution at room temperature for 24 h,
followed by calcination at 1073 K for 6 h. Figure 3 shows AFM
and KFM images of a calcined zirconia thin film. Both images
confirm the formation of a zirconia thin film confined in nano-
wires. These results show that the present method is available
for patterning zirconia thin films.
This study demonstrates the patterning of zirconia thin films
using templates fabricated by self-assembly in phase-separated
mixed LB films. To the best of our knowledge, this is the first
report on the directed self-assembly of inorganic thin films
reflecting the phase-separated structures of mixed LB films
through sol–gel processes. The structures of the zirconia films
can be designed by controlling the original phase-separated
structures of the mixed LB films. The present method will be
important in the fabrication of patterned materials on the micro-
meter/nanometer length scale for applications to memories,
switches, and sensors, because various metal ions can be used
as precursors.
Figures 2a and 2b shows AFM and KFM images, respective-
ly, of the film calcined at 1073 K for 6 h. The circular domains
are higher than the rest in contrast to the structure of the zirco-
nia/SAM composite film shown in Figure 1c. In addition, the
chemical species in the domains and the other part are different
because the KFM image shows that they have different
surface potentials. A functionalized template calcined under
the same condition showed smooth surface due to the burning
off of the SAMs of EDA–Si and F8H2SiCl. These results indi-
cate that the products confined in the domains remain even
after the calcination, while the SAMs of EDA–Si and F8H2SiCl
are removed from the substrate. Scanning electron microscopy
(SEM) observation and the elemental analysis using energy dis-
persive X-ray (EDX) spectroscopy were carried out to determine
the chemical species of the products in the domains. The SEM
image showed a patterned surface similar to that of the phase-
separated structure of an as-deposited LB film with circular do-
mains shown in Figure 2, confirming the existence of products in
the circular domains. A peak assigned to Zr11 was clearly ob-
served in the EDX spectra of the domain region. In contrast,
the EDX spectra of the other part showed no peak assignable
to Zr. These results confirmed that the patterned zirconia thin
film was prepared on the templates fabricated from phase-sepa-
rated LB films.
Finally, we succeeded in confining zirconia thin films on
templates having patterns on a nanometer length scale. We fab-
ricated mixed LB films of stearic acid (H17A), 10, 10, 11, 11, 12,
12, 13, 13, 14, 14, 15, 15, 16, 16, 17, 17, 17-heptadecafluorohep-
tadecanoic acid (F8H8A) and (heptadecafluoro-1,1,2,2-tetrahy-
drodecyl)triethoxysilane (F8H2SiOEt) at mixing ratios of 2, 2
to 5. Mixed LB films of this type have phase-separated structures
of nanowire patterns with nanowires consisting of H17A and
F8H8A surrounded by F8H2SiOEt.10 Templates were fabricated
and functionalized as described above. Zirconia thin films were
fabricated by immersing the functionalized templates in 360 mM
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Published on the web (Advance View) October 13, 2007; doi:10.1246/cl.2007.1356